As a measure to prevent electrostatic breakdown caused by electrostatic pulses from input and output terminals, it has been known to use n well-p substrate diodes or MOSFET parasitic diodes as protective devices elements.
FIG. 3 is a cross sectional view showing a diode portion of a known semiconductor device having a p substrate-n well diode. An n well region 311 is formed in a surface layer of a p substrate 301, to provide a p substrate-n well diode 312. A cathode electrode 306 is formed in contact with a high concentration n cathode region 304 that is formed in the n well region 311, and this electrode 306 is connected to input and output terminals (not shown). A high concentration p.sup.+ pickup region 302 is formed in another surface portion of the p substrate 301, and an anode electrode 307 is formed in contact with a surface of the pickup region 302 such that the electrode 307 is grounded. Reference numeral 310 denotes an oxide film.
FIG. 4 is a cross sectional view showing a diode portion of another known semiconductive device having MOS diodes. An n well region 411 is formed in a surface layer of a p substrate 401, and p source region 420 and p drain region 419 are formed in the n well region 411. A gate electrode layer 422 made of polycrystalline silicon is formed on a gate oxide film 426 over a surface portion of the n well region 411 between the p source region 420 and p drain region 419, to thus provide a p channel MOSFET. A high concentration n.sup.+ pickup region 421 and the p source region 420 have a common source electrode 423 formed thereon. In this known example, the source electrode 423 is also connected to the gate electrode layer 422. Also, a p well region 413 is formed in another portion of the surface layer of the p substrate 401, and n source region 414 and n drain region 415 are formed in the p well region 413. A gate electrode layer 416 made of polycrystalline silicon is formed on a gate oxide film 427 over a surface portion of the p well region 413 between the n source region 414 and n drain region 415, to thus provide an n channel MOSFET. A high concentration p.sup.+ pickup region 402 and the n source region 414 have a common source electrode 417 formed thereon. In this known example, the source electrode 417 is also connected to the gate electrode layer 416. A drain electrode 418 that is in contact with both the p drain region 419 of the p channel MOSFET and the n drain region 415 of the n channel MOSFET is connected to input and output terminals. The p well region 413 and n drain region 416 constitute a p well-n drain diode 424, and the p drain region 419 and n well region 411 constitute a p drain-n well diode 425. These diodes 424, 425 provide protective elements for preventing electrostatic breakdown caused by electrostatic pulses from the input and output terminals.
The p substrate-n well diode 312 of FIG. 3 has high internal resistance, and therefore a large diode area is required for sufficiently absorbing electrostatic pulses from the input and output terminals. Although the MOS diodes of FIG. 4 can deal with positive and negative pulses by use of the n channel MOSFET and p channel MOSFET, both of the p well-n drain diode 424 and p drain-n well diode have high internal resistance, and therefore require large diode areas, as in the case of the p substrate-n well diode of FIG. 3 as described above. During use of the semiconductor device of FIG. 4, a four-layer structure consisting of the p drain region 419, n well region 411, p well region 413 and n source region 414 may latch up in some cases.